EC: Add the DN side Reconstruction Handler class.

The idea here to provide the Class Structure and direction for reconstruction work. 

When ECReconstructionContainersCommand arrives at DN, it will delegate the call to ReconstructECContainersCommandHandler.

Here it will extract the required information from ECReconstructionContainersCommand and create ECReconstrctionInfo. With this abstraction, we will create Task thread( ECReconstructionCoordinatorTask) and passed to a simple ECReconstructionSupervisor service. This service responsibility is that, it will simple execute the tasks given to it.

So, for each ECReconstructionCoordinatorTask, that itself should take care of providing full implementation of actual reconstruction work. 

In side the Task run method, provide the outline proposed implementation.